1. Field of the Invention
The present invention relates to a method for measuring a charge/voltage conversion ratio of a solid state image pickup element with an electronic shutter function.
2. Description of the Prior Art
In a solid state image pickup element, a charge/voltage conversion ratio can be obtained from a ratio between a current value and a voltage value since a charge value can be represented as a current value in case of taking a time factor into consideration. Now, the charge/voltage conversion ratio represents an efficiency in case of converting transfer charges into a voltage at an output portion of the solid state image pickup element.
It has been proposed in the art to provide such a measuring method for the conversion ratio that, in a circuit arrangement of an output portion of the solid state image pickup element, e.g., a charge-coupled device (CCD) image pickup element shown in FIG. 1, a current I.sub.RD flowing through a reset drain (RD) terminal 42 of a gate circuit 40 is measured when a positive pulse .phi..sub.R is applied to a reset gate (RG) terminal 41 of the gate circuit 40 constituted by a MOS FET which constitutes a floating diffusion amplifier (FDA), then a change in signal charges being stored in a diode 43 in an off state of the gate 40 is detected as a change in a voltage by an amplifier 44 constructed by two stages of source follower circuits to thereby measure an output voltage V.sub.OUT applied to an output terminal 45, and further a charge/voltage conversion ratio .eta. is obtained based on the thus measured current I.sub.RD and voltage V.sub.OUT in accordance with a following formula EQU .eta.=(V.sub.OUT /I.sub.RD).times.K1
where K1 is a coefficient.
According to this measuring method, a dedicated terminal for the measurement is not required to be provided independently, and the measurement is performed while operating the CCD image pickup element in a normal operation mode, so that the measurement can be performed while the CCD image pickup element being in both a wafer state and an assembled state.
However, in the above-mentioned conventional measuring method, since the current I.sub.RD flowing through the RD terminal 42 is intermittently interrupted by the high-frequency pulse .phi..sub.R to be a current similar to an AC current, the measurement is required to measure an integrated current value obtained by smoothing the current I.sub.RD similar to the AC current, so that a value measured by a measuring unit such as an ampere meter is likely to include an error. In particular, since the integrated current is such a small current of about several nA, then a leakage current in the measuring unit likely influences a measured value to thereby increase a measuring error. Further, a reproduced image may be disturbed by connecting the ampere meter to the RD terminal 42.
In recent years, a small-sized image pickup element is decreased in its number of terminals in order to miniaturize a chip such that the RD terminal 42 is also used as a power source terminal 46 to which a power supply voltage V.sub.DD same as the voltage V.sub.RD is applied to thereby decrease the number of terminals. The conventional measuring method can not be applied to such an image pickup element wherein the RD terminal 42 is also used as the power source terminal 46 since the conventional measuring method requires that the RD terminal 42 is independently provided.